1. Field of the Invention
The invention relates to a circuit arrangement for A/D conversion of two color difference signals of a first picture signal and/or a carrier-modulated chrominance signal of a second picture signal.
2. Description of the Related Art
For example, if two chrominance signals are to be processed in a television receiver, both the luminance signals of these picture signals and their chrominance information components must be converted to the digital domain. If, for example, the picture of one picture signal is to be inserted into the picture of the other picture signal, the luminance signals of the two picture signals can be alternately applied to an A/D converter, for example, via a switch. In the further digital signal processing of these luminance signals the switch-over between the two luminance signals does not lead to any problems. However, this is different particularly when processing a chrominance signal at the carrier frequency. Such a carrier-frequency chrominance signal must be converted to the baseband by means of a demodulator. However, such a demodulator has considerable transients when a signal is only temporarily available. Similar problems occur when the chrominance information components of the two picture signals are available as chrominance signals at the carrier frequency. Thus, it is necessary that in such a case the relevant chrominance signal to be demodulated is continuously applied to the demodulator. For this reason it is not possible, or at least detrimental, to switch over before the A/D converter, for example, between the two color difference signals of the one picture signal and the carrier-modulated chrominance signal of the other picture signal. In fact, in this case the demodulator would only temporarily receive the carrier-modulated chrominance signal so that transients which are clearly visible on a display screen will result after each switching process.
In known color television receivers, this problem is solved in that the chrominance information components of the two picture signals have their own A/D converter. However, since particularly A/D converters involve high costs, such a solution is not very economical.